Extract nullAssignmentError

convert.go

@@ -369,7 +369,7 @@ func NullAssignTo(dst interface{}) error {
 
 	// AssignTo dst must always be a pointer
 	if dstPtr.Kind() != reflect.Ptr {
-		return errors.Errorf("cannot assign NULL to %T", dst)
+		return &nullAssignmentError{dst: dst}
 	}
 
 	dstVal := dstPtr.Elem()
@@ -380,7 +380,7 @@ func NullAssignTo(dst interface{}) error {
 		return nil
 	}
 
-	return errors.Errorf("cannot assign NULL to %T", dst)
+	return &nullAssignmentError{dst: dst}
 }
 
 var kindTypes map[reflect.Kind]reflect.Type

pgtype.go

@@ -3,6 +3,7 @@ package pgtype
 import (
 	"database/sql"
 	"encoding/binary"
+	"fmt"
 	"math"
 	"net"
 	"reflect"
@@ -198,6 +199,14 @@ func (f BinaryEncoderFunc) EncodeBinary(ci *ConnInfo, buf []byte) (newBuf []byte
 var errUndefined = errors.New("cannot encode status undefined")
 var errBadStatus = errors.New("invalid status")
 
+type nullAssignmentError struct {
+	dst interface{}
+}
+
+func (e *nullAssignmentError) Error() string {
+	return fmt.Sprintf("cannot assign NULL to %T", e.dst)
+}
+
 type DataType struct {
 	Value Value